1. Field of the Invention
The invention relates to a method of operating an engine. More specifically, it relates to a method for operating an engine to allow evaporative emission purge during operation of an engine in a dual combustion mode.
2. Discussion of the Related Art
Internal combustion engines can be coupled to fuel vapor recovery systems to allow purging of fuel vapors generated in the fuel tanks. Conventional systems control total purge flow based on an estimated purge fuel vapor concentration representing the amount of fuel vapor contained in the total purge flow from the fuel system. Fuel injection control is also adjusted based on this estimated purge concentration.
The present inventor developed a method for accurately determining the cylinder fuel vapor purge concentration. This invention was disclosed in U.S. application Ser. No. 09/366,124, and its disclosure is incorporated herein by reference. By knowing the cylinder fuel vapor purge concentration as well as the concentration in the fuel vapor recovery system, control of the operation of the engine can be accomplished. The inventor also has developed a method for determining the overall fuel vapor purge concentration. This invention was disclosed in U.S. application Ser. No. 09/055,500, and its disclosure is also incorporated herein by reference.
Direct injection spark ignition (DISI) engines can operate in multiple modes of operation. These modes can include a homogeneous mode where the air-fuel mixture is injected during the intake stroke; a stratified mode where additional fuel can be added during the compression stroke; and a combined or dual mode that allows injection on the intake stroke and further injection of fuel during the compression stroke. The combined mode is especially useful for high load conditions and knock resistance.
Multiple injections can also used to make transitions between the stratified and homogeneous modes and to extend the envelope of non-homogeneous operation.
In the dual mode, it is difficult to control the proper amount of fuel so that it is balanced and will not produce unburned hydrocarbons or over fuel/over torque the engine. Currently there is no way to control the total quantity of fuel burned to provide a desired torque output from the engine and to balance the amount of fuel supplied by each of the various sources (canister purge vapor, fuel injected during the intake stroke, fuel injected during the compression stroke) so that the homogeneous portion of the cylinder charge is at an air-fuel ratio that provides reliable combustion and acceptable emissions when ignited and the stratified portion of the charge provides reliable ignition as well as providing the final portion of fuel necessary to deliver the desired torque. Further, there is currently no method for maximizing the usage of fuel vapors from the purge system.
None of the prior art allows for a balanced control of the engine in the dual mode when the carbon canister is feeding purge vapor to the engine cylinders. Typically, if the purge fuel vapor is added to the engine in the prior art structures, the mixture is often too lean to burn and the fuel is wasted. In some cases, the unburned hydrocarbons are used to warm up the catalyst but this usually increases emissions and does not provide a balanced control to solve the above-mentioned problems.
An object of the present invention is to use the fuel vapor concentration to control the operation of the engine, especially when the engine is operating in dual mode.
It is a further object of the present invention to provide a method that uses purge flow from the carbon canister with a minimum impact on fuel economy and pollution.
During stratified operation of a DISI engine, the fuel is injected during the compression stroke which creates a region of combustible mixture in only a portion of the cylinder which is surrounded by a non-combustible mixture of air and exhaust residuals. In the dual mode of operation, fuel is injected on the intake stroke to provide a lean, nearly homogeneous air fuel mixture throughout the cylinder, then additional fuel is injected during the compression stroke to create a region of richer mixture that will be easier to ignite with the spark plug.
If fuel vapors from the carbon canister are introduced to the intake manifold, this fuel will be mixed throughout the cylinder. During stratified operation, the portion of the evaporative fuel that is in the rich region will burn. The fuel in the remainder of the cylinder will burn (and produce torque) only if it is rich enough.
If the overall mixture is too lean, it will not burn and this results in higher feed gas hydrocarbon emissions and increased catalyst temperatures. Also, this fuel will be consumed without producing useful work resulting in a lower fuel economy. If the overall mixture is near the lean limit of flammability, it may burn during some combustion events and not burn during others and this will result in rough engine operation. Further, if the canister is not purged during stratified operation, it can be overfilled and inadequately purged during typical operation. However, if the engine is run in a homogeneous mode more often to allow for more canister purge, the fuel economy benefit from stratified operation is reduced.
In the dual mode, it is desirable to control the total quantity of fuel burned to provide a desired torque output from the engine and further, to balance the amount supplied by each of the various sources (canister purge vapor, fuel injected during the intake stroke, fuel injected during the compression stroke) such that the homogenous portion of the cylinder charge is at and air-fuel ratio that provides reliable combustion (and acceptable emissions) once ignited and the stratified (richer) portion of the charge provides reliable ignition as well as providing the final portion of the fuel necessary to deliver the desired torque. Also, maximizing the usage of fuel vapors from the evaporative purge system is desired to maximize the purge of the carbon canister during vehicle operation. To provide the proper balance, a total fuel quantity desired is determined; this fuel is then apportioned between fuel supplied during the intake stroke and fuel supplied during the compression stroke. Further, the fuel supplied during the intake stroke is comprised of fuel vapors from the carbon canister and fuel injected during the intake stroke.
In general if Ft is the total fuel desired, then the sum of Fp (the fuel that is available from the canister purge system), Fh (the fuel injected during the intake stroke) and Fs (the fuel injected during the compression stoke) should equal Ft:
Fp+Fh+Fs=Ft.
Further, in the homogenous portion of the charge, given a mass of air in the cylinder (MA) and a desired homogenous air fuel ratio (AFh), the fuel supplied during the intake stroke should satisfy the equation:
MA/(Fp+Fh)=AFh.
The resulting air fuel ratio measured in the exhaust system (AFx) would be:
AFx=MA/(Fp+Fh+Fs)
Alternately, when it is adequate for the homogenous portion of the charge to simply be within a range of air fuel ratios (AFhMin, AFhMax) then:
AFnMin less than MA/(Fp+Fh) less than AFhMax
where AFnMin would be a minimum combustible air fuel ratio, and AFhMax would be the maximum air fuel ratio desired for acceptable emissions.
If the fuel quantity that will be supplied by the carbon canister is not currently known, the homogenous fuel can initially be supplied by injection. Then as flow from the carbon canister is induced the homogenous injected fuel is reduced to maintain the desired overall air fuel ratio. The amount of reduction provides the information necessary to determine the fuel content of the purge flow. Conversely, once the fuel content of the purge is known, a purge flow can be commanded that will provide as much fuel as possible without exceeding the desired homogenous air fuel ratio. If the fuel from the purge is insufficient to provide the desired, combustible, air fuel ratio additional fuel would be injected during the intake stroke to make up the deficit.
The basic operation of the method of operating the engine includes the following steps. Determine whether the canister purge is desired based on the time since the purge was active, or if the purge is currently active, continue purging the canister if the estimated purge fuel composition contains more fuel than some predetermined value. Given the current engine speed and torque requirements, determine a target air charge or manifold pressure for semi-stratified or dual mode operation. If the purge composition (fuel fraction) is known, then determine the purge flow required to deliver the required homogeneous fuel. If the fuel flow that is required exceeds a predetermined flow capability, determine the fuel available at the maximum purge flow and then calculate how much additional fuel needs to be injected during the intake stroke (if this is below a minimum quantity that can be reliably injected, reduce purge flow to allow for a minimum injection quantity or pulse width).
A feedback from an exhaust sensor can be used to refine the estimate of the purge composition similar to the current practice.
If the purge composition is not known, all of the homogeneous fuel can be provided via the intake stroke injection. Then the amounts of purge flow introduced into the intake manifold can be gradually increased while modifying the quantity of fuel injected during the intake stroke to maintain the desired total quantity of fuel using a sensor in the exhaust system for feedback. The purge composition can be determined based on the estimated volume of the purge flow and the magnitude of the change required in the injected fuel that is similar to the current practice.
Finally, the remaining fuel that must be injected to form a richer portion of the cylinder charge is determined. The remaining fuel is determined by subtracting the homogeneous fuel quantity from the total fuel required for the desired engine operation.
The objects of the present invention are achieved by a method of operating an engine comprising the steps of: allowing purge flow from the fuel vapor canister to a cylinder of the engine; supplying purge flow and a first injected fuel to the cylinder of the engine during an intake stroke and supplying a second injected fuel during a compression stroke so that the engine operates in a dual mode including both homogeneous operation and stratified charge operation; and adjusting the purge flow and the first injected fuel supplied to provide a predetermined amount of homogeneous fuel during the supplying steps so as to balance an amount of fuel supplied during the intake stroke and the compression stroke.
The objects of the present invention can also be achieved by a method for operating an engine, comprising the steps of includes determining a fuel fraction in a fuel vapor canister; determining whether the fuel vapor canister should be purged; performing purge flow from the fuel vapor canister to a cylinder of the engine; supplying purge flow and injected fuel to a cylinder of the engine during an intake stroke and supplying injected flow during a compression stroke so that the engine operates in the combined homogeneous and stratified charge mode; controlling the purge flow and the injected fuel supplied so as to balance an amount of fuel supplied during the intake stroke and the compression stroke, and calculating the amount of fuel to be injected during the intake stroke and during the compression stroke, and adjusting the amount of fuel supplied during the intake stroke based on an amount of purge supplied to the cylinder.
An advantage of the present invention is the ability of using the evaporative emission purge during dual mode operation of the DISI engine by making it sufficiently rich to burn.
Another advantage of the present invention is to allow the canister to be purged even during extensive lean operations without reverting to operation in the homogeneous mode that will lower fuel economy.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.